Expanding your home is a big decision that requires careful consideration and planning. Among the different steps to follow, creating a solid and durable foundation for your house extension is essential. This article presents practical advice for properly preparing and successfully completing your foundation work.
Soil study before construction
Before starting your foundation work, it is essential to carry out a geotechnical study of the ground on which your extension will be built. This analysis makes it possible to determine the nature of the terrain, its load-bearing capacity and the possible risks involved (landslides, floods, etc.).
The results of the study are then used to design the type of foundation suited to your project. For example, if the ground has a low level of bearing capacity, it will be necessary to opt for a deep foundation which is supported on posts or micropiles. Conversely, on stable ground, we can be satisfied with a classic superficial reinforced concrete foundation.
Choice of foundation type
Superficial foundations
Shallow foundations are most commonly used for house extensions as they are less expensive and quicker to install. They rest on a concrete slab poured directly on the ground and reinforced by reinforced concrete beams to ensure the solidity of the whole. Several types of superficial foundations exist:
- The strip sole: ideal for load-bearing walls, it is made by digging a continuous trench under the entire length of the wall, then pouring reinforced concrete inside.
- The sill: similar to the strip sole, it however rests on foundation pads at the intersections of the load-bearing walls to further reinforce the structure.
- Insulated supports: they are used to support supporting posts or specific structural elements, such as a staircase.
Deep foundations
Deep foundations are necessary when the ground is not sufficiently load-bearing or presents a geotechnical risk (for example, in flood-prone areas). They consist of anchoring the extension structure on piles or micropiles driven to a sufficient depth to reach layers of stable soil. Two main methods exist:
- Drilled piles: their installation requires the use of a drill which digs holes in the ground, up to the desired depth. Concrete is then poured into the holes to create the piles, which will be connected to the extension structure by reinforced concrete beams.
- Screw piles: this method uses screw-shaped metal piles, which are screwed directly into the ground using a machine. The piles are then fixed to the structure by reinforced concrete or metal beams.
Costs and budget
The cost of foundation work essentially depends on the type chosen (superficial or deep) and the surface area of the extension. A superficial foundation will generally be less expensive, with an average cost of between 100 and 200 €/m². For a deep foundation, prices are more variable and can reach up to €500/m² or more, depending on the complexity of the site and the geographical location.
To best estimate the total cost of your work, it is recommended to request several quotes from qualified professionals, who can advise you on the type of foundation best suited to your project.
Compliance with regulations
When considering building a house extension, it is important to comply with current regulations. Depending on the size of your project, you will need to obtain a building permit or make a prior declaration of work to your town hall.
Depending on the geographical location of your land, specific constraints may also apply (coastal, flood-prone, seismic zones, etc.). Do not hesitate to contact your municipality and consult the Local Urban Plan (PLU) to find out the rules applicable to your project.
Tips for successful foundation work
To successfully complete the foundations of your house extension, here are some tips to follow:
- Have a geotechnical study of the soil carried out: this will allow you to choose the type of foundation suitable for your land.
- Consult several professionals: ask for several quotes and compare prices, but also the services and references of the companies consulted.
- Plan a realistic budget: do not underestimate the cost of the work and allow a safety margin to deal with possible unforeseen events.
- Comply with the regulations in force: check the legal obligations applicable to your project and carry out the required administrative procedures before starting the work.
- Ensure the quality of the implementation: monitor the site and do not hesitate to ask questions of the company carrying out the work if you have any doubts or questions.
By following these tips and taking the time to plan your project carefully, you will put all the chances on your side to successfully carry out the foundation work on your house extension.
